High-frequency oven door seal



y 20, 19.55 P. J. DE VRIES ETAL 3, 96,242

HIGHFREQUENCY OVEN DOOR SEAL Filed 001;. 24, 1962 2 Sheets-Sheet 1 INVENTOR PEKE J.DE VRIES JACOBUS B.V|SSER AG EN July 1965 P. J. DE VRIES ETAL 3,196,242

HIGH-FREQUENCY OVEN DOOR SEAL 2 Sheets-Sheet 2 Filed 001,. 24 1962 .llrnllunllllllll flilltllrlifiiriltlilllrlliill llflln llulllldllilll la FIG. 3

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United States Patent 0 3,1%,2 l2 HIGELFREQUENCY OVEN DOOR SEAL Peke Jan de Vries and Jaeobus Bernardus Vlsser,

Drachten, Netherlands, assignors to North American Philips Company Inc, New York, N .Y., a corporation of Delaware Filed Oct. 24, 1962, Ser. No. 232,732 Claims priority, application Netherlands, 0st. 25, 1961, 27d,6ll$ 3 Claims. (ill. 2l9-=-lt).55)

The invention relates to a high-frequency oven for high-frequency heating food with the aid of ultra highfrequency oscillations, for example oscillations in the decimeter or centimeter range, which comprises an ultrahigh-frequency generator the output circuit of which is coupled to a heating chamber enclosed by metal walls and provided with an aperture for introducing the articles to be heated into the oven and with a door which in the closed position closes the heating chamber and. forms a seal against emanation of stray radiation.

In such high-frequency ovens for use in the home or a restaurant particular attention must be paid to providing an effective seal against high frequency stray radiation, since such stray radiation may injure the operators retina. The upper limit of the admissible power of the emanating stray radiation is set at 10 rnw./cm.

It is known to reduce the stray radiation by designing the door wall situated outside the heating chamber together with the closing wall provided on the heating chamber as 21 M4 seal, however, in this arrangement no complete protection against the various wave types (modes) occurring in the heating chamber is obtained owing to the wavelength-dependent character of the door seal.

In another high-frequency furnace endeavours are made to provide a seal by establishing a metallic contact between the door wall and the closing wall, for example, by the provision of a resilient metal or metallizedrubber sealing ring, however, the e is no certainty that the emanating stray radiation is maintained below the limit value because owing to soiling, for example, by deposition of fat during the preparation of food, a satisfactory metallic contact is seriously impaired.

To ensure an effective seal for the emanating stray radiation to maintain the limit value thereof below 10 mw./cm. special requirements have to be satisfied which are considerably enhanced by the additional requirements of practical use. In the above-mentioned seals, for example, the door Wall and the closing wall always must accurately engage one another for, if owing to an impurity or damage a slit-shaped aperture having a width of only a few millimeters is produced, the stray radiations emanating through this aperture rises to a multiple of the admissible limit value. Further the sealing construction must be readily accessible for effective cleaning in order to satisfy the requirements of hygiene and this is achieved only partly in the known constructions due to the shape employed.

An object of the present invention is to provide, in a high-frequency furnace of the type described above a new concept of sealing with respect to emanating stray radiation which not only has remarkable simplicity of construction permitting simple and effective cleaning but also ensures effective sealing under all operating conditions.

The arrangement in accordance with the invention is characterized in that at least one of the sealing walls situated outside the feed aperture in a direction pointing away from this feed aperture comprises two successive sealing frames surrounding the feed aperture which in the closed position of the door engage the opposite wall so as to be substantially flush therewith, the sealing frame adjoining the feed aperture consisting of metal and the following sealing frame consisting of a dielectric material in which is distributed a powdered substance which absorbes ultra high-frequency oscillations.

Suitable dielectric materials are, for example, synthetic materials, especially synthetic resins, for example epoxy resins, urea or phenolic resins and polyester resins, and ceramic materials, while especially powdered graphite when used as the material absorbing the ultra high-frequency radiation provides excellent results.

The invention and its advantages will now be described in detail with reference to the figures.

FIGURE 1 is a vertical cross sectional view of the high-frequency furnace in accordance with the invention, while FlGURES 2 and 3 are a partial view of a front elevation of the sealing frames provided on the closing wall of the oven and a vertical sectional view showing the sealing construction of FIG. 2 in greater detail.

The high-frequency oven in accordance with the invention comprises a metal housing 1 which is horizontally divided into two compartments 2 and 3, the upper compartment 2 comprising a heating chamber 4 enclosed by metal walls while the lower compartment 3 contains an ultra high-frequency generator in the form of a magnetron generator 5 and a control panel 6 with the associated control instruments and control knobs. The lower compartment ".5 in addition to the magnetron generator 5, which is rated at, for example 2 kw. and designed to produce oscillations having a wavelength of 12 cms. (2450 mc./s.), contains the electric apparatus associated with the magnetron generator 5 such as, for example, transformers, rectifiers, voltage stabilizers and the like, which are not shown in the figure for the sake of clarity.

In the high-Z'equency oven shown the output circuit of the magnetron generator 5 is coupled to a wave guide 7 for the transmission of the ultra high-frequency oscillations produced, one end of the wave guide '7 being connected to an input coupling aperture 8 in the upper wall of the heating chamber 4 while in order to provide load matching for the magnetron generator 5 the other end is provided with matching means, for example, in the form of a known matching piston 9.

The front wall of the high-frequency oven is provided with a feed aperture it for articles 11 to be heated, which are introduced into the heating chamber 4 on a dielectric plate 12 of loss-free material, and with a door 14 which is operated with the aid of a handle 13 and in the closed position closes the heating chamber 4 by its metal front wall. In the embodiment shown the door 14 is hinged to the housing wall by a curved hinge member 15 and is provided with a locking mechanism 16 which locks the door 14 to the casing wall in the closed position. As is shown in the figure, the door 1 3 when closed is flush with the housing of the high-frequency furnace.

Uniformity of the dielectric heating is promoted by a field stirrer 1'7 which is arranged in the wave guide 7 over the input coupling aperture 8 and is driven by a motor not shown in the figure. To reduce any stray radiation emanating between the door and the walls of the heating chamber l an outwardly extending closing Wall surface is provided in the plane 18 on said chamber 4 which together with the inner wall of the door 14 forms a seal against stnay radiation in a manner described more fully hereinafter.

The invention provides a remarkably simple and particularly effective sealing construction in that a closing wall in plane 18 situated in the plane of the feed aperture it) and extending in a direction pointing away from said aperture 19 comprises two successive or concentric sealing frames 19 and Eli which surround the feed aperture ill and in the closed position of the door 14, engage the adjacent wall of the door 14 so as to be flush therewith, the sealing frame I9 adjoining the aperture 10 consisting of metal and the following sealing frame 2t consisting of a dielectric material in which a powdered substance is distributed which absorbs the ultra high-frequency oscillations. In the embodiment described the dielectric material of the sealing frame 20 is an epoxy resin (araldite) and the material absorbing the ultra high-frequency radia- .tion energy is powdered graphite.

FIGURES 2 and 3 show, side by side, a front elevation of the sealing frames 19 and 2% comprising on the closing wall 18 and a cross-sectional view of the sealing construction used which, as will be described more fully hereinafter, ensures an effective reduction of the stray radiation emanating from the high-frequency furnace.

If in the high-frequency furnace shown the ultra-highfrequency oscillations set up in the heating chamber 4 are to emanate from the high-frequency furnace, they have to pass through the narrow gap between the metal sealing frame 19 adjoining the feed aperture ft and the inner wall of metal door 14 and when travelling in a direction pointing away from the feed aperture 10 they are guided by the metal walls of the sealing frame 19 and the door 14- after the manner of a wave guide and at the dielectric sealing frame 2%) they are dispersed in a direction at right angles to the gap between the sealing frame 19 and the door wall, since at this point the metallic wave guide is interrupted, The larger part of the emanating radiation penetrates into the dielectric sealing frame 2%! and this effect is enhanced by the fact that compared with the dielectric constant of air the dielectric constant of the dielectric material is large, for example, about 3, and the ultra high-frequency radiation penetrating into the sealing frame 26 is absorbed by the powdered graphite distributed therein. The metal sealing frame 19 adjoining the feed aperture in and the door wall not only form a first seal but also cause decoupling of the heating chamber 4 from the dielectric sealing frame 2h which, as described hereinbefore, ensures that the ultrahigh-frequency radiation is not only dispersed but also absorbed.

Thus in the sealing construction described by using a powdered absorbing material in a dielectric medium in the sealing frame 28 an effective reduction of the ultra high frequency stray radiation was obtained, while it has been found that this reduction hardly occurs when the sealing frame 2t) is made of pure graphite but assumes an optimum value at a suitable ratio between the powdered graphite and the dielectric material, especially if in the frequency range under consideration approximately from 20% to 40% of the weight is graphite. In the embodiment described, in which approximately 30% of the weight is powdered graphite having a grain size of from 50-100 microns, the emanating stray radiation is reduced to a value far below the limit value of l mw./cm. the emanating stray radiation in this case being only approximately 1 mw./crn. at a distance of cms. from the edge of the door 14.

Instead of an epoxy resin other dielectric materials may be used for the sealing frame 29, for example, synthetic resins such as urea and phenolic resins (Philite) and polyester resins, and ceramic material, while powdered graphite provides particularly satisfactory results when used as the material absorbing the ultra high-frequency radiation. With all these synthetic resins a same percentage by weight of the powdered absorbing material proves to give optimum conditions, for example a percentage by weight of approximately from 20%-40% when powdered graphite is used.

' The sealing construction described not only is level and hence permits of simple and effective cleaning but also proves to be only slightly dependent upon the widthof the gap between the two sealing frames 1.9 and 2t) and the inner wall of the door 14, and this is apparently due to the fact that increasing the width of the gap increases the absorbing effect of the sealing frame 2% by increasing the dispersing effect. Thus when the gap width was increased to 1.5 mms. the emanating stray radiation was only at most 7 mw./cm. at a distance of 5 c-rns. from the edge of the door 14.

Under all operating conditions, for example, in the case of impurity or damage the said sealing construction ensures an effective suppression of the emanating stray radiation and this property is' also of particular importance for the manufacture of the high-frequency furnaces under consideration because no special tolerance requirements need to be satisfied.

The following data are given of a high-frequency furnace extensively tested in practice:

Cms. Dimensions of the heating chamber 4 6'1 x 37 x '41 Dimensions of the feed aperture 10 61 x 37 Width of the sealing frame 19 2.5 Width of the sealing frame 26 5.5 Thickness of the sealing frame 2% 2.0

In a further development of the seal described it has been found of advantage for the face of the dielectric sealing frame 2d, remote from the door 14, to be open in order to intercept any radiation having passed through the said sealing frame 2d in the enclosed space bounded by the walls of the housing 1 and the heating chamber 4 where it can do no damage. In the embodiment shown slit-shaped apertures 22 are provided for this purpose in a metal support 21 of. the sealing frame 20.

An additional safety step may be to provide on the dielectric sealing frame 20 a microswitch (not shown) in the form of a resilient pin which, when a maximum permissible width of the gap between the sealing frame ing chamber, means for supplying high frequency energy to said chamber, a feed aperture in said chamber for introducing food articles to said chamber, a metallic door for closing said opening, a metallic decoupling seal surrounding said feed aperture and having a planar contact surface in the plane of said aperture engageable with said door in the closed position thereof, a successive nonconductive dielectric seal concentrically surrounding and directly adjoining said metallic seal, said successive di+ electric seal having a contact surface engageable with said door in the plane of said aperture, said second seal consisting of a dielectric material containing 20-40 percent by weight of powdered graphite distributed therethro-ugh for dispersing and absorbing said energy existing between said door and said seals.

I 2. A high frequency oven door seal comprising a heatmg chamber, means for supplying high frequency energy to said chamber, a feed aperture in said chamber for introducing food articles to said chamber, a metallic door for closing said opening, a metallic decoupling seal surrounding said feed aperture and having a planar contact surface in the plane of said aperture engageable with said door in the closed position thereof, a successive nonconductive dielectric seal concentrically surrounding and directly adjoining said metallic seal, said successive dielectric seal having a contact surface engageable with said door in the plane of said aperture, said second seal consisting of a dielectric material having powdered graphite distributed therethrough for dispersing and absorbing said energy existing between said door and said seals, said successive dielectric seal having an unobstructed surface opposite said contact surface communicating with a space defined by an oven enclosure and the walls of said heating chamber.

3. A high frequency oven door seal comprising a heating chamber, means for supplying high frequency energy to said chamber, a feed aperture in said chamber for introducing food articles to said chamber, a metallic door for closing said opening, a metallic decoupling seal surrounding said feed aperture and having a planar contact surface in the plane of said aperture engageable With said door in the closed position thereof, a successive nonconductive dielectric seal concentrically surrounding and directly adjoining said metallic seal, said successive dielectric seal having a contact surface engageable with said door in the plane of said aperture, said second seal consisting of a dielectric material containing 2040 percent by weight of powdered graphite distributed therethrough for dispersing and absorbing said energy existing between said door and said seals, said successive dielectric seal having an unobstructed surface opposite said contact surface communicating with a space defined by an oven enclosure and the walls of said heating chamber.

References Cited by the Examiner UNITED STATES PATENTS ANTHONY BARTIS, Examiner. 

1. A HIGH FREQUENCY OVEN DOOR SEAL COMPRISING A HEATING CHAMBER, MEANS FOR SUPPLYING HIGH FREQUENCY ENERGY TO SAID CHAMBER, A FEED APERTURE IN SAID CHAMBER FOR INTRODUCING FOOD ARTICLES TO SAID CHAMBER, A METALLIC DOOR FOR CLOSING SAID OPENING, A METALLIC DECOUPLING SEAL SURROUNDING SAID FEED APERTURE AND HAVING A PLANAR CONTACT SURFACE IN THE PLANE OF SAID APERTURE ENGAGEABLE WITH SAID DOOR IN THE CLOSED POSITION THEREOF, A SUCCESSIVE NONCONDUCTIVE DIELECTRIC SEAL CONCENTRICALLY SURROUNDING AND DIRECTLY ADJOINING SAID METALLIC SEAL, SAID SUCCESSIVE DIELECTRIC SEAL HAVING A CONTACT SURFACE ENGAGEABLE WITH SAID DOOR IN THE PLANE OF SAID APERTURE, SAID SECOND SEAL CONSISTING OF A DIELECTRIC MATERIAL CONTAININT 20-40 PER- 